The invention teaches a transmission bridge for a subscriber's circuit comprising a supply source, supply resistors and terminals for the connection of a subscriber's line, the supply source being coupled, in a polarity independent manner, to the connecting terminals via the supply resistors.
Such a transmission bridge comprising a transformer having two primary windings, arranged in series by means of a capacitor, and at least one secondary winding, the supply source being connected to the connecting terminals via a polarity-reversing switch the supply resistors and the primary windings are generally known.
This known transmission bridge has the property that a.c. voltage signals, so-called differential mode signals such as speech and ringing signals, occurring in anti-phase at the connecting terminals, are transferred from the primary to the secondary side and vice versa by means of the transformer and that in-phase a.c. voltage signals, so-called common mode signals, such as unwanted signals induced by a noise source in the two wires of the subscriber's line, occurring on the connecting terminals are blocked by the transformer and attenuated by the supply resistors.
In addition, this known transmission bridge is of a symmetrical structure so that common mode signals are prevented from being converted into differential mode signals and vice versa. Because of the transformer such a transmission bridge is rather bulky. Furthermore, a portion of the differential mode signals is applied across the capacitor connected between the primary windings and, consequently, across the supply resistors, causing these signals to be attenuated.
It is an object of the invention to realize a transmission bridge described in the preamble, which, while retaining said advantages such as the polarity independance can be implemented wholly electronically and wherein substantially no attenuation of the differential mode signals by the supply resistors occurs.